Question

import inspect
class Test:
  def test(self, p, d={}):
    d.update(p)
    return d
print inspect.getargspec(getattr(Test, 'test'))[3]
print Test().test({'1':True})
print inspect.getargspec(getattr(Test, 'test'))[3]

I would expect the argspec for Test.test not to change but because of dict.update it does. Why?

Answer 1

Because dicts are mutable objects. When you call d.update(p), you are actually mutating the default instance of the dict. This is a common catch; in particular, you should never use a mutable object as a default value in the list of arguments.

A better way to do this is as follows:

class Test:
    def test(self, p, d = None):
        if d is None:
            d = {}
        d.update(p)
        return d